1. Field of the Invention
This invention relates to a liquid crystal display panel for a liquid crystal display device of the type used extensively in various apparatuses such as watches and hand-held calculators, particularly to a liquid crystal display panel that has color filters or a reflecting films or whose liquid crystal layer consists of a mixture of a liquid crystal and a polymer obtained by imparting a crosslinked structure to a monomer by ultraviolet light exposure.
2. Description of the Related Art
A liquid crystal display panel is constituted by disposing two glass panels or other transparent material substrates to oppose each other across a gap, providing one of the opposed substrate surfaces with multiple display electrodes and the other with multiple transparent counter electrodes to form a large number of pixel regions in matrix arrangement between the display electrodes and the counter electrodes, and filling a liquid crystal layer between the two substrates.
Images are displayed by selectively applying voltages between the display electrodes and the counter electrodes of the individual pixel regions so as to modify an optical characteristic of the intervening liquid crystal.
In the simplest structure of this type of liquid crystal display panel, the signal electrodes for inputting the display signal also serve as the display electrodes and a simple matrix of pixel regions is formed at the intersections between the signal electrodes and the counter electrodes. A multiplex drive system is adopted.
Most liquid crystal display panels in use today are of the active matrix type in which a display electrode is formed separately of the signal electrode at each picture element and a switching element is provided between each signal electrode and the associated display electrode.
Switching elements fall in two general categories, the three-terminal type using a thin film transistor (TFT) and the two-terminal type using a non-linear resistance element. A diode, varistor, thin film diode (TFD) or the like is used as the non-linear resistance element.
In addition, wide use is being made of color liquid crystal panels having color filters of three colors on one of the substrates and reflective liquid crystal display panels having a reflecting film on one of the substrates. Moreover, a liquid crystal display panel has been developed that uses a liquid crystal layer consisting of a mixture of a liquid crystal and a polymer obtained by crosslinking a monomer by exposure with ultraviolet light.
The liquid crystal display device using this type of liquid crystal display panel has the advantage of consuming much less power than other display devices.
While it is therefore used extensively as the display device of various kinds of equipment, particularly portable equipment, the display capacity of the liquid crystal display panel has increased steadily in recent years.
At the same time, demand for brighter displays has made it necessary to enhance the transmittance of the individual pixel regions and to increase the aperture ratio, i.e., the ratio of the pixel regions to the gaps between the pixel regions (non-display regions).
Although an attempt has also been made to increase display luminance by utilizing difference of refractive index in liquid crystal, this leads to change in hue depending on the positional relationship between the liquid crystal display panel and the observer and the relationship between the position of the pixel regions and the observer.
An example of the structure of a conventional color liquid crystal display panel using two-terminal type non-linear resistance elements will now be explained with reference to FIGS. 29 and 30.
FIG. 29 is an enlarged partial plan view of the liquid crystal display panel. For ease of illustration, some elements such as the second substrate, an insulated protection layer and an alignment film are omitted and lower members among members that overlap in the vertical direction are also indicated by solid lines in the plan views. FIG. 30 is an enlarged sectional view taken along line A--A in FIG. 29.
The first substrate 1 and the second substrate 6 shown in FIG. 30 are transparent substrates such as glass panels. They are disposed opposite and parallel to each other.
A large number of signal electrodes 2 made of tantalum (Ta) film are formed on the first substrate 1 in the pattern of regularly spaced strips, as shown in FIG. 29. Each signal electrode 2 is integrally formed at regular intervals along its longitudinal direction with laterally projecting first electrodes 2a. A non-linear resistance layer 3 made of tantalum oxide (Ta.sub.2 O.sub.5), an anodic oxide film of the signal electrode 2, is provided on each signal electrode 2 and its first electrodes 2a.
Second electrodes 4 made of chromium (Cr) film are provided to overlap the non-linear resistance layer 3, thereby constituting non-linear resistance elements 10.
A large number of reflecting film display electrodes 15 made of aluminum (Al) are formed on the first substrate 1 in a closely spaced matrix arrangement. Portions of the second electrodes 4 make contact with the display electrodes 15. As shown in FIG. 29, the first electrodes 2a and the display electrodes 15 are separated by a prescribed distance. Each display electrode 15 is disposed in alignment with a counter electrode 9 across an intervening liquid crystal layer 16 to constitute a pixel region 19 of the liquid crystal display panel.
On the other hand, as indicated by the oblique lines in FIG. 29, a two-layer black matrix 7 composed of a chromium oxide (CrO) film and a chromium (Cr) film is formed in a cross stripes pattern on the surface of the second substrate 6 opposed to the first substrate 1. The black matrix 7 serves as a light-shielding film for preventing light leakage from the gaps between the display electrodes 15 formed on the first substrate 1.
Color filters 11, 12 and 13 that partially overlie the black matrix 7 are provided on the second substrate 6 at regions opposite the display electrodes 15 (regions completely covering the pixel regions 19). These filters 11, 12 and 13 are of three colors: blue (B), red (R) and green (G).
The counter electrodes 9 are further provided on the second substrate 6 as strips of indium-tin-oxide (ITO) film, which is transparent and conductive, running perpendicular to the signal electrodes 2 so as each to oppose one column of the display electrodes 15. Data electrodes (not shown) are connected to the counter electrodes 9 for applying signals from an external circuit. Transparent and insulated protection layers 8 are provided between the counter electrodes 9 and the color filters 11, 12 and 13.
The opposed inner surfaces of the first substrate 1 and the second substrate 6 are provided with alignment films 21A, 21B as processed layers for regularly aligning the molecules of the liquid crystal.
A prescribed spacing is maintained between the first substrate 1 and the second substrate 6 by means of spacers 17. The liquid crystal layer 16 is filled in the intervening gap. A polarization film 18A is disposed on the outer (lower) surface of the first substrate 1 and a polarization film 18B is disposed on the outer (upper) surface of the second substrate 6.
Since this liquid crystal display panel is not self-illuminating, it uses the external light 28 (natural or artificial light) from the side of the second substrate 6 for display.
Specifically, an optical characteristic of the liquid crystal in the regions between the display electrodes 15 and the counter electrodes 9 is modified via the non-linear resistance elements 10 by selectively applying driving voltages produced by an external circuit across the signal electrodes 2 and (via the data electrodes) the counter electrodes 9. By this, display of a desired image can be effected by using the modification of the liquid crystal optical characteristic (including, for example, rotation of the major axes of the liquid crystal molecules) to control the exiting, through different ones of the color filters 11, 12 and 13 and the polarization film 18B, of the external light entering the liquid crystal display panel through the polarization film 18B from the side of the second substrate 6 and reflected by the display electrodes 15.
In this type of color liquid crystal display panel the color filters 11, 12 and 13 provided at the individual pixel regions 19 are one-piece filters each covering the whole of one pixel region 19. In addition, the color filters 11, 12 and 13 partially overlap the black matrix 7 provided at the peripheral portions of the pixel regions 19.
This means that all of the external light 28 entering the liquid crystal display panel passes through different ones of the color filters and that all of the light reflected by the display electrodes 15 exits through different ones of the color filters.
The display therefore exhibits good color purity. On the other hand, however, since the light transmittance of the color filters is only around 60-70% and the light has to pass through the color filters twice before exiting, the amount of exiting light, i.e., the amount of light contributing to the display, is considerably reduced relative to the amount of incident light. The display is proportionally dark.
Moreover, in order to obtain a visible display even when the external light 28 is weak, it is desirable to dispose an auxiliary light source on the side (on the rear side) of the first substrate 1 opposite from that formed with the display electrodes 15. Even if such an auxiliary light source is provided, however, the illumination it provides from the rear surface of the liquid crystal display panel cannot enable an acceptably visible display since almost all of the incident light from the auxiliary light source is blocked by the reflecting films constituting the display electrodes 15.
The display luminance of the conventional color liquid crystal display panel is therefore low and the displayed image becomes substantially unrecognizable when the external light is weak. In addition, the degree to which an auxiliary light source can be used to overcome this problem is extremely limited owing to the color filters provided on the side of one substrate and the reflecting films provided as the display electrodes on the side of the other substrate.
The luminance of an active matrix type liquid crystal display panel having non-linear resistance elements is still lower than that of a simple matrix type liquid crystal display panel in which the intersection regions between the counter electrodes and the signal electrodes constitute the pixel regions, because the regions formed with the non-linear resistance elements block the passage of light.
Moreover, when a polymer dispersive liquid crystal composed of a liquid crystal-polymer mixture is to be used as the liquid crystal layer, the monomer of the liquid crystal layer 16 shown filled in the gap between the first substrate 1 and the second substrate 6 in FIG. 30 has to be exposed with ultraviolet light from the side of the first substrate 1 or the second substrate 6 in order to effect the crosslinking reaction needed to obtain a polymer with a crosslinked structure. However, almost no irradiation with ultraviolet light is possible from the side of the first substrate 1, since most of the first substrate 1 is formed with the reflecting films constituting the display electrodes 15. On the other hand, efficient exposure of the liquid crystal layer 16 with ultraviolet light is impossible from the side of the second substrate 6 since the ultraviolet light (wavelength: 300-400 nm) is absorbed by the color filters 11, 12 and 13 provided on the second substrate 6, particularly by the blue color filter 11, which passes almost no ultraviolet light.
Irradiation with ultraviolet light is also required when using a liquid crystal layer composed of a mixture of a polymer dispersive liquid crystal and a dye. Efficient exposure of the liquid crystal layer with ultraviolet light is also impossible in this case.